1. Field of the Invention
The present invention relates generally to a communication system employing HSDPA (High Speed Downlink Packet Access), and in particular to an apparatus and a method for transmitting/receiving an uplink transmission power offset value and a DSCH (Downlink Shared Channel) power level.
2. Description of the Related Art
In general, HSDPA (High Speed Downlink Packet Access) refers to a technique for transmitting data including HS-DSCH (High Speed-Downlink Shared Channel), a downlink data channel, for supporting high-speed packet data transmission, and a control channel related to the HS-DSCH in an UMTS (Universal Mobile Telecommunications System) communication system. In order to support the HSDPA, AMC (Adaptive Modulation and Coding), HARQ (Hybrid Automatic Retransmission Request), and FCS (Fast Cell Select) have been proposed.
First, the AMC will be described. The AMC is a data transmission technique for adaptively determining a modulation technique and a coding technique of different data channels according to a channel condition between a Node B and a UE, thereby to increase the overall utilization efficiency of the Node B. Therefore, the AMC has a plurality of modulation techniques and a plurality of coding techniques, and modulates and codes data channel signals by combining the modulation techniques and the coding techniques. Generally, each of combinations of the modulation techniques and the coding techniques is called “MCS (Modulation and Coding Scheme)”, and MCSs with level #1 to level #n can be defined according to the number of MCSs. In other words, the AMC adaptively determines a level of the MCS according to a channel condition between the Node B and the UE currently wirelessly connected to the Node B, thereby increasing the overall efficiency of the Node B.
Second, the HARQ, especially n-channel SAW HARQ (Stop And Wait Hybrid Automatic Retransmission Request) will be described. The HARQ has introduced the following two plans to increase transmission efficiency of ARQ (Automatic Retransmission Request). A first plan is to exchange a retransmission request and a response between a UE and a Node B. A second plan is to temporarily store defective data and combine it with corresponding retransmitted data before transmission. Further, the HSDPA has introduced the n-channel SAW HARQ in order to make up for the shortcomings of the conventional SAW ARQ (Stop And Wait ARQ). In the SAW ARQ, a Node B does not transmit the next packet data until ACK (Acknowledgement) for the previously transmitted packet data is received. Therefore, in some cases, the Node B must await ACK, though it can presently transmit packet data. The n-channel SAW HARQ increases channel utilization efficiency by continuously transmitting a plurality of data packets before receiving the ACK for the previous packet data. If n logical channels are established between a UE and a Node B and identified by time or channel numbers, the UE, upon receipt of packet data at a certain time point, can determine the logical channel that transmitted the packet data. Thus the UE can rearrange packet data in the right reception order or soft-combine the packet data.
Finally, the FCS will be described. The FCS is a technique for rapidly selecting a cell having a good channel condition among a plurality of cells, when a UE supporting the HSDPA enters a cell-overlapping region, or a soft handover region. To be specific, if the UE supporting the HSDPA enters a cell-overlapping region between an old Node B and a new Node B, then the UE establishes radio links to a plurality of the cells, i.e., a plurality of Node Bs. A set of the cells, to which the radio links are established by the UE, is called an “active set.” The UE receives HSDPA packet data from only the cell maintaining the best channel condition among the cells included in the active set to reduce the overall interference. Herein, a cell transmitting the HSDPA packet data for its best channel condition among the cells in the active set is called a “best cell.” The UE periodically checks the channel conditions with the cells belonging to the active set. Upon detecting a cell having a channel condition better than that of the current best cell, the UE transmits a best cell indicator to all of the cells in the active set in order to replace the current best cell with a new best cell. The best cell indicator includes an identifier of the selected new best cell. Upon receiving the best cell indicator, the cells belonging to the active set analyze the cell identifier included in the received best cell indicator to determine whether the received best cell indicator is destined for them. The selected best cell transmits packet data to the UE using HS-DSCH.
As described above, in the HSDPA, it is necessary to exchange the following new control signals between a UE and a Node B in order to support the newly introduced AMC, HARQ, and FCS. First, in order to support the AMC, a UE must provide information on a channel condition between the UE and a Node B to the Node B, and the Node B must inform the UE of an MCS level determined according to the channel condition using the channel information received from the UE. Second, in order to support the n-channel SAW HARQ, a UE must transmit an ACK or NACK (Negative Acknowledgement) signal to a Node B. Third, in order to support the FCS, the UE must transmit to the Node B a best cell indicator indicating a best cell, i.e., a Node B providing a channel with the best channel condition. In addition, if the best cell is changed according to the channel condition, the UE must inform the Node B of its packet data reception state at that point, and the Node B should provide necessary information so that the UE can correctly select the best cell.
FIG. 1 schematically illustrates a downlink channel structure of a general communication system employing HSDPA. Referring to FIG. 1, a downlink DPCH (Dedicated Physical Channel) includes a field defined in an existing CDMA (Code Division Multiple Access) communication system, e.g., Release-99, and an HS-DSCH indicator (HI) indicating presence/absence of HSDPA packet data to be received at a UE. The HS-DSCH indicator transmitted over the downlink DPCH informs a corresponding UE of the presence/absence of HSDPA packet data to be received. Further, in the presence of the HSDPA packet data, the HS-DSCH indicator informs the UE of a channelization code for SHCCH (Shared Control Channel) that it must receive control information for HS-DSCH over which the HSDPA packet data is actually transmitted. In addition, if necessary, a part of the HS-DSCH control information, e.g., an MCS level, can be transmitted through the HS-DSCH indicator.
For example, in the case where the HSDPA packet data is transmitted for a period of N (=N1+N2) slots (i.e., HSDPA TTI (Transmission Time Interval)=N slots), if a slot structure remains unchanged for the TTI, the HS-DSCH indicator is separately transmitted for N1 slots and a section for transmitting the HS-DSCH indicator for the remaining N2 slots is subject to discontinuous transmission (DTX). It is assumed in FIG. 1 that the HS-DSCH indicator is transmitted for one slot, i.e., N1=1.
A Node B then transmits information for controlling HS-DSCH (hereinafter, referred to as HS-DSCH control information), such as an MCS level, HS-DSCH channelization code, HARQ processor number and HARQ packet number, to the UE over a SHCCH (SHared Control CHannel). A description of the HS-DSCH control information will be described herein below.
(1) MCS level: this indicates a modulation technique and a channel coding technique to be used by HS-DSCH.
(2) HS-DSCH channelization code: this is a channelization code used for a specific UE by HS-DSCH.
(3) HARQ processor number: when the n-channel SAW HARQ is used, this indicates a channel to which a specific packet belongs among logical channels for HARQ.
(4) HARQ packet number: when a best cell is changed in the FCS, this informs a UE of a unique number of a downlink packet data so that the UE can inform a selected new best cell of a transmission state of the HSDPA data.
The SHCCH can be allocated either a single channelization code, or two or more channelization codes. The HS-DSCH is a channel over which HSDPA packet data is transmitted from the Node B to the UE. In FIG. 1, a start point of the downlink DPCH precedes start points of the SHCCH and the HS-DSCH, because the UE cannot recognize whether the remaining two channels are data corresponding to the UE itself, before reading the HS-DSCH indicator to detect corresponding information. Therefore, the UE must temporarily store the data in a buffer, so it receives the remaining two channels allowing a sufficient time to read the HS-DSCH indicator, thereby to lighten a load of the UE buffer. As a result, the UE determines whether there exists HSDPA packet data to receive by reading the HS-DSCH indicator on the downlink DPCH. If there exists HSDPA packet data to receive, the UE reads HS-DSCH control information on the SHCCH and then receives the HSDPA packet data over HS-DSCH according to the control information.
FIG. 2 illustrates a downlink DPCH structure of a general communication system employing HSDPA. Referring to FIG. 2, a downlink DPCH has a structure of a downlink DPCH defined in an existing CDMA communication system not employing the HSDPA, e.g., defined in Release-99, and the structure has the following fields. A Data1 field and a Data2 field transmit data for supporting an operation of an upper layer, or data for supporting a voice-only service. A TPC (Transmission Power Control) field transmits a downlink TPC command for controlling uplink transmission power, and a TFCI (Transmission Format Combination Indicator) field transmits TFCI information of the Data1 and Data2 fields. A Pilot field is a field for transmitting a pilot symbol stream previously defined by the system, and is used by a UE to estimate a downlink channel condition. HS-DSCH indicator for the HSDPA service, as illustrated in FIG. 2, is transmitted to the UE through a newly defined field in an existing Release-99 downlink DPCH channel structure.
FIG. 2 shows a case where the HS-DSCH indicator is transmitted through a newly defined field in the existing downlink DPCH. However, FIG. 3 shows a case where the HS-DSCH indicator is transmitted over a new downlink DPCH instead of a specific field in the existing downlink DPCH.
FIG. 3 illustrates another downlink DPCH structure of a general communication system employing HSDPA. Referring to FIG. 3, the HS-DSCH indicator is transmitted over a new downlink DPCH that has been allocated a separate channelization code, instead of a specific field in the existing downlink DPCH. Two downlink DPCHs, i.e., a primary DPCH (P-DPCH) and a secondary DPCH (S-DPCH), are allocated. Since the S-DPCH for transmitting the HS-DSCH indicator is different from the P-DPCH in an amount of transmission data, the P-DPCH is allocated a spreading factor (SF) N and the S-DPCH is allocated an SF M. If the HS-DSCH indicator to be transmitted has a small data amount, the SF value M of the S-DPCH is set to a relatively large value, e.g., M=512, thus to increase utilization efficiency of a downlink channelization code.
FIG. 4 illustrates an uplink DPCH structure of a general communication system employing HSDPA. Referring to FIG. 4, DPDCH (Dedicated Physical Data Channel) and DPCCH (Dedicated Physical Control Channel) for supporting the existing CDMA communication system, e.g., Release-99, and HS-DPCCH (High Speed Dedicated Physical Control Channel) for supporting the HSDPA are allocated separate channelization codes, and then transmitted independently. In the case of an uplink, since all UEs are allocated unique OVSF (Orthogonal Variable Length Spreading Factor) codes, channelization code resources are sufficient. If modified, the existing uplink control channel may not be compatible with the existing system and an increase in complexity of its structure would be required. Therefore, it is preferable to define a new uplink control channel using a new separate channelization code instead of modifying the channel structure.
Upper layer data from a UE and a Node B is transmitted over slots constituting one uplink DPDCH frame, and slots constituting one uplink DPCCH frame each include Pilot symbol, TFCI symbol, feedback information (FBI) symbol, and TPC symbol. The Pilot symbol is used as a channel estimation signal when demodulating data transmitted from the UE to the Node B. The TFCI symbol indicates a transmission format combination (TFC) used for data transmission by the channels transmitted for a current frame. The FBI symbol transmits feedback information when a transmission diversity technique is used. The TPC symbol is a symbol for controlling transmission power of a downlink channel. The uplink DPCCH is transmitted after being spread using an OVSF code, and an SF (Spreading Factor) used at this point is fixed to 256.
In the HSDPA, a UE performs error checking on data received from a Node B, and transmits ACK or NACK for the received data according to the error checking result. The ACK and NACK are transmitted over HS-DPCCH for supporting the HSDPA. If the UE is not required to transmit ACK/NACK to the Node B since there is no received data, the UE transmits channel quality information (CQI) to the Node B over the HS-DPCCH in order to support the AMC, or transmits other information such as a best cell indicator indicating a Node B that provides the UE with the best channel, over the HS-DPCCH in order to support the FCS. As illustrated in FIG. 4, when the HS-DPCCH for the HSDPA service is allocated a separate channelization code, it undergoes the same transmission power control as the existing DPCCH. That is, the DPCCH and the HS-DPCCH have a specific power ratio, so if transmission power of the DPCCH is increased or decreased, transmission power of the HS-DPCCH is also increased or decreased.
Next, the AMC technique for HS-DSCH will be described with reference to FIGS. 5A to 5C.
FIGS. 5A to 5C illustrate an AMC technique for HS-DSCH in a general communication system employing HSDPA. Specifically, FIG. 5A illustrates a signal constellation for QPSK (Quadrature Phase Shift Keying) modulation. The QPSK modulation, as illustrated in FIG. 5A, is a technique for making two transmission bits into one complex signal. For example, this technique modulates bits “00” into a complex signal “1+j.” Here, since 4 complex signals are located on a circle centered on its origin, they have the same transmission power level. A receiver performs demodulation on a QPSK-modulated signal depending on a quadrant to which the QPSK-modulated signal belongs, among 4 quadrants formed by X-axis and Y-axis on the signal constellation. For example, if a received QPSK-modulated signal exists in a first quadrant, a transmission signal is demodulated into bits “00.” That is, in the QPSK modulation, decision lines of a transmission signal are X-axis and Y-axis.
FIGS. 5B and 5C illustrate a signal constellation of 16QAM (16-ary Quadrature Amplitude Modulation) for modulating/demodulating 4 transmission bits into one complex signal, wherein FIG. 5C is larger than FIG. 5B in a channel gain of HS-DSCH. Since FIG. 5C is larger than FIG. 5B in the channel gain of the HS-DSCH, distances of complex signals from an origin on the signal constellation of FIG. 5C become longer than distances of complex signals from an origin on the signal constellation of FIG. 5B. The 16QAM modulates 4 bits into one complex signal on the signal constellation, and a 16QAM-modulated signal is demodulated according to a decision boundary formed by dotted lines, or decision lines in FIGS. 5B and 5C. As illustrated in FIGS. 5A to 5C, since a 16QAM-modulated signal has different decision lines during demodulation according to its channel gain, a receiver must recognize a channel gain of the transmitter in order to demodulate the 16QAM-modulated signal. Of course, in the QPSK, since the decision line is fixed regardless of transmission power, the receiver can perform demodulation even though it does not recognize a channel gain of the transmitter. Therefore, N-ary QAM requires a process of transmitting control information indicating a channel gain from a transmitter or Node B to a receiver or a UE. That is, the control information related to the channel gain transmitted from the Node B to the UE is called an “HS-DSCH power level,” and the HS-DSCH power level is defined as a ratio of HS-DSCH power for one code to CPICH (Common Pilot Channel) power (or defined as a power difference in a dB unit). The HS-DSCH power for one code is power that can be allocated to a specific UE identified by a specific channelization code, among the whole transmission power allocated for the HSDPA service.
FIG. 6 illustrates a method of determining an HS-DSCH power level in a general communication system employing HSDPA. Referring to FIG. 6, in order to express the HS-DSCH power level with P bits, available transmission power of HS-DSCH for one code is divided into 2P areas defined from transmission power 0 to CPICH power. In FIG. 6, in order to express the HS-DSCH power level with 2 bits, the HS-DSCH power level is divided into four areas (1) to (4). For example, if HS-DSCH transmission power for one channelization code belongs to an area (2), a Node B sets an HS-DSCH power level to A, and transmits bits “10” indicating the HS-DSCH power level A over a downlink. In general, since CPICH must be transmitted to every cell, CPICH power is much greater than HS-DSCH power for one channelization code. Therefore, if a difference between HS-DSCH power for one channelization code and CPICH power is large, a plurality of transmission bits are required in order to correctly express an HS-DSCH power level. Accordingly, there is a need for a method of determining an HS-DSCH power level by the Node B in order to demodulate a QAM-modulated signal from the UE. In addition, there is a demand for a method of transmitting information on the HS-DSCH power level to the UE.
If the DPCCH and the HS-DPCCH are transmitted (or controlled) in a specific power ratio as described in conjunction with FIG. 4, a transmission power problem may occur. This will be described with reference to FIG. 7.
FIG. 7 schematically illustrates a channel allocation scheme for a UE located in a soft handover region in a general communication system employing HSDPA. In the channel allocation scheme of FIG. 7, one UE is located in a soft handover region where it receives a service from K Node Bs. Even though the UE travels to the soft handover region while receiving an HSDPA service from a Node B#1, it does not necessarily receive the HSDPA service from all Node Bs including new Node Bs. That is, if a channel condition is not good while continuously receiving packet data from the Node B#1, the UE informs another Node B with a best channel condition, i.e., a best cell of the packet data transmission by the UE itself, and thereafter performs a hard handover in which it receives an HSDPA service from a new Node B with the best channel condition after disconnecting the connection with the Node B#1. As a result, the UE receives packet data for the HSDPA service from only one Node B. However, a voice service undergoes an existing soft handover in which a UE maintains connections with several Node Bs, so the UE, as illustrated in FIG. 7, receives channels for the HSDPA service from the Node B#1, and receives channels for the voice service, i.e., existing Release-99 DPCHs, from all Node Bs (Node B#2 to Node B#K) in the soft handover region. In addition, the UE transmits DPDCH and DPCCH to all Node Bs over an uplink, but transmits HS-DPCCH containing HSDPA service-related information such as ACK/NACK only to the Node B#1 from which it receives the HSDPA service.
Transmission power control over a Node B employing the existing Release-99 by a UE is performed as follows. A Node B measures SIR (Signal-to-Interference Ratio) through a pilot symbol on an uplink DPCCH, and compares the measured SIR with a target SIR. As a result of the comparison, if the measured SIR is less than the target SIR, the Node B transmits a power-up command for uplink transmission power to the UE over a TPC field on an uplink DPCH. In contrast, if the measured SIR is greater than the target SIR, the Node B transmits a power-down command for uplink transmission power to the UE over the TPC field on the uplink DPCH. Here, a measured SIR less than the target SIR means that the channel condition is bad, so the Node B transmits a power-up command for uplink transmission power. In contrast, that the measured SIR greater than the target SIR means that the channel condition is relatively good, so the Node B transmits a power-down command for uplink transmission power.
In FIG. 7, the UE also controls the uplink channel transmission power as in the Release-99. To be specific, if there is any power-down command for uplink transmission power among uplink transmission power control commands transmitted over TPC fields on downlink DPCHs from all Node Bs, the UE decreases uplink transmission power. For example, in the case where an uplink channel environment for a Node B#1 is bad, although the Node B#1 gives a power-up command for uplink transmission power to the UE, if any one of other Node Bs except the Node B#1 transmits a power-down command for uplink transmission power to the UE, the UE will decrease uplink transmission power. Therefore, though the Node B#1 providing the HSDPA service continuously issues a power-up command for uplink transmission power as illustrated in FIG. 7, transmission power of an uplink DPCCH may be decreased due to other Node Bs, and transmission power of HS-DPCCH that undergoes power control while maintaining a specific power ratio to the uplink DPCCH may also be reduced.
If the UE is located in a soft handover region, uplink DPDCH and DPCCH for the Release-99 are transmitted to all Node Bs and combined in an upper layer, obtaining a soft handover effect. In this case, event though transmission power is decreased to some extent, no problem occurs. However, since the HS-DPCCH of FIG. 4 transmitting ACK/NACK necessary for the HSDPA service or other control information for the HSDPA service is transmitted only to a single Node B, i.e., the Node B#1, a decrease in its uplink transmission power leads to a decrease in reliability.